The disposal of improperly stored and discarded hazardous solids is a major and worldwide environmental problem. Some of these improper disposal sites are extremely large with U.S. sites such as New Bedford Harbor and the Hudson River having millions of tons of polychlorinated biphenyl (PCB) and oil contaminated sediments. Worldwide, tens of thousands of sites are estimated to require cleanup with multimillion dollar per site cleanup costs being common.
This existence of large numbers of extensively contaminated sites has created a major need for low cost methods of treating contaminated solids.
Direct treatment methods such as incineration and dechlorination (Peterson Pat. No. 4,574,013, Rogers et al. Pat. No. 5,019,175) are very effective for cleaning up solids contaminated with organics. However, such methods are relatively expensive and generally operate at elevated temperatures (150.degree. to 1500.degree. C.).
To reduce the cost, contaminated solids may be pretreated before direct methods are applied.
Soil washing and solvent extraction are well-known methods for pretreating contaminated solids. Soil washing refers generally to processes using water as the primary solvent while solvent extraction implies a use of solvents other than water. In processing contaminated soils, soil washing has been extensively used in Europe for sandy solids; but "treatment of high clay soils has not been proven" (NTIS PB89-212656 "Cleaning Excavated Soil using Extraction Agents: A State of the Art Review", p. 15). In general, the European soil washing procedures are limited to materials with a maximum of 20-30% fines (&lt;63 micron) (NTIS PB90-106428 "Assessment of International Technologies for Superfund Applications: Technology Review and Trip Report Results").
Soil washing processes such as those of Trost (U.S. Pat. No. 4,783,263), Giguere (U.S. Pat. No. 4,336,136) and Yoshida (U.S. Pat. No. 4,555,345) generally use water amended with caustics and surfactants such as soaps to cause the oily contaminants to separate from the clean solids. The oil and contaminated fines are removed by froth flotation. Solvent extraction processes such as those of Weitzman (U.S. Pat. No. 4,662,948), Keane (U.S. Pat. No. 4,610,729), Steiner (U.S. Pat. No. 4,801,384) and Morris (U.S. Pat. No. 4,606,774) use volatile chlorinated solvents or light hydrocarbon solvents to dissolve contaminants; and they then use standard separation methods such as filtration to separate the contaminated liquids from the clean solids.
In soil washing and solvent extraction, large volumes of solvent or water, generally ten to fifteen times the volume of the solids, are required to achieve effective extraction. This is a major drawback of these processes since the amount of material handled strongly affects the overall processing costs.
Thus, there is an existing and continuing need for a low cost method of pretreating contaminated solids and thereby reducing the cost of subsequently treating those solids by direct methods such as incineration and dechlorination. Further needed is a decontamination process of that character which generates less waste than washing and solvent extraction techniques.